Now, if you're a 5th-level wizard, you can cast Knockdown once per day. If you're a 15th-level wizard, you can cast Lockdown once per day. So we expect that Knockdown will have roughly the same average impact against a 5th-level ogre that Lockdown has against a 15th-level storm giant.
Actually, you raise a good point here, even if it isn't the one you were trying to make. Lower level spells have less effect. It probably should be easier to Knockdown an enemy than Lockdown them.
Perhaps scaling DC based on level should exist, but should go in reverse. That way, choosing to use a fireball at 3rd level makes it harder to resist but does less damage.
The current proposal is to return to the 3E system, albeit with (I hope!) less drastic DC scaling. This is, at best, a solution to one problem that introduces another problem. I certainly don't see any overriding necessity for going with this as opposed to the AD&D approach.
I'm not sure what problem it introduces. I've read your message and I don't see one.
The 3e system works just fine to fix the problem you are talking about. If you are a 5th level Wizard and casting Knockdown on Ogres, their bonuses to save combined with your bonuses to DC give them, as an example, a 60% chance to save against Knockdown.
If you are level 15 and casting Lockdown on a Storm Giant, your bonuses to DC along with their bonuses to save mean they have a 60% chance to save against Lockdown. Therefore Lockdown DOES have the same chance to work on Storm Giants as Knockdown does on Ogres. That's why saves and DCs need to scale at the same rate.
It, on the other hand, has the added benefit of allowing your the spells that aren't Save or X to work better against high level monsters.
For instance, if you are level 10 and cast fireball in 2e against 10 HD creatures, they need an 11 to save vs spell. So the average damage is 26.25 points of damage with a fireball at this level. Meanwhile, a hypothetical Knockdown spell has a 50% chance of affecting 10 HD creatures.
Once you get to level 20, enemies need a 6 to save vs spells. That means the average damage of a fireball is now 21.875. Which means you managed to gain 10 levels but now do less damage with your spells even though your enemies have more hitpoints. You also only have a 25% chance to affect 20 HD creatures with a hypothetical Knockdown spell OR a Lockdown spell.
Also, even though you are now 10 levels above where you were before, you STILL only have a 50% chance of effecting 10 HD creatures with Knockdown and your average damage against them with fireball is still exactly the game, so you haven't even gotten better at defeating low level creatures with any spell you knew while you were low level.
It hurts my suspension of disbelief to get more powerful but to have all my spells get less powerful or stay exactly the same as I go up levels. I like the idea that I'm so powerful at 20th level that my spells now have an 80% chance of effecting weak creatures instead of the 50% chance they had before.